Web Services changed how Email+ calculates email open counts. When reporting is turned on we place a hidden image within each email. When we detect that the image is displayed we add a record to our database indicating that a person has opened the email. But some people don't display images by default within their email client. For those people we were never recording an open email.
We have modified our software such that when someone clicks a link we check to see if we have recorded an open email within the last 30 minutes. If we have not then we record an "opened email" record to our database. This should increase the number of open emails being recorded.